There are a lot more protocols that would benefit from the more generic and much simpler solution of "define a standard port where the service listens over TLS".
IRCS has settled on 6697: https://tools.ietf.org/html/rfc7194
IMAPS uses 993.
IRCS has settled on 6697: https://tools.ietf.org/html/rfc7194
IMAPS uses 993.
That said, dedicated SMTPS port would still be beneficial by cutting down at least 2 RTTs (HELO/EHLO + STARTTLS) from overall transaction time.